With the increasing use of high power transistors (1 to 80 Watts) in electrical circuits which operate at high frequencies, such as radio frequencies (RF), e.g. 25 MHz to 1GHz, it has become necessary to design transistor packages which permit the transistors to operate properly in such circuits. The packages for such high power transistors must be capable of dissipating the relatively high quantities of heat generated by the transistor to maintain the transistor at a suitable operating temperature, i.e., a temperature which will not adversely affect the operating characteristics of the transistor. Another important characteristic for such packages is that they provide for good electrical grounding of the transistor with a minimum of parasitic inductance.
In order to satisfy these requirements and provide collector isolation, prior art high frequency semiconductor device packages for many high frequency transistors utilize internal metallized ceramic insulators. The metallization provides electrical connection to the transistor while the ceramic provide electrical isolation but thermal conduction between, typically, the transistor collector and the underlying heat sink on which the device is ultimately mounted.
For example, U.S. Pat. No. 3,969,752 discloses a semiconductor device package having a metal layer for supporting the bottom of an NPN transistor die and connecting to the transistor, collector, and having a ceramic insulator underlying the collector metal layer and overlying the lowest metal layer functioning as a ground terminal and as a heat sink. The bottom of the transistor die is the collector. This is because a top collector transistor configuration causes large parasitics. However, the collector must be separated from the common or ground terminal which is usually connected to the emitter. Therefore, electrical isolation of the bottom of the transistor from the ground is needed. Another U.S. Pat. No. 3,728,589 describes a semiconductor assembly having a mounting block of an electrical insulating but thermally conducting material, such as beryllium oxide or alumina, mounted in a cavity in a ground plane member.
Although the heat dissipation and the electrical parasitic characteristics of RF transistor device packages have been improved in the prior art, they require additional materials and processes for introducing metallized ceramics, which are time and cost consuming. Insulating BeO and Al.sub.2 O.sub.2 ceramics 40 to 60 mils thick, such as are commonly used in the prior art, are expensive. Further, the ceramic must be metallized, with e.g. gold, and then usually brazed or soldered to other package components. As a result, the final package cost including the cost of the ceramic material, the metallization, brazing or soldering, and the heat sink becomes quite high.
In addition, some prior art semiconductor device packages are prone to have flux and solder enter the cavity of the device, for example, during soldering operations used to attach the packages to the circuit. Further, most prior art RF packages are not well suited for surface mounting and are not designed to be easily handled by pick and place equipment. Also, positioning of capacitors close to most prior art packages is often difficult because of the protruding flying leads found on these packages.